1) Mill off flat in a drill press with a router bit.. the rubber part of your existing Honda CR250 intake manifold.

2) Get a stock off the shelf 2 inch or about 51mm diameter universal Mikuni intake rubber boot intake adapter that has the same diagonal Honda bolt spacing as the OEM intake has.

New Update to step 2:
I Found the Mikuni rubber intake manifold adapter was not robust enough to stand the riggers of 250 superkart racing.
I had a very serious ..very hard to find vacuum leak at the Mikuni carb intake adapter as shown in the inc. photos

..So don’t use a rubber adapter like I did at first!
I had to weld up a better replacement unit made of all steel using an old salvaged garbage disposal outlet plate and a piece of 1&3/4 inch tail pipe as shown in the inc. photo.

This then is attached to the previously made stock Honda intake manifold using 2 flat head 6mm internal bolts and 2 old salvaged standard shifterkart 428 chain links drilled out a bit to 6mm for extra strength added to the top and bottom of the newly made intake piece at the outside corners as shown intake inc. photo.

Then you may proceed to step 3.

3) Use just part of a common Gates 21236 rubber lower radiator hose …for now almost a straight shot with no tight bends like the stock Honda boot had and larger (1&3/4 inch or 44.5mm i.d.) diameter then the stock boot had… to help clear the Honda kickstarter big case bump. Put a 1 & 11/16 inch or 43mm diameter short tube spacer inside the intake end of the hose to reinforce and stiffen the rubber hose under the hose clamp.

Note: This slightly longer intake run.. did not seem to hurt the 250’s performance one bit.

4) Purchase and install a PWK 41mm Pro Series Keihin / Sudco modified slide carburetor # 016.156AD from Amazon. We set up our carb with 178 high speed jet and a 60 low speed jet to start off with… and used the same needle the carb came with too.

5) Purchase and install a SharkShifter CR125 Dellorto fuel pump mount. This does require re-drilling the mount to fit the CR250.

6) Purchase and install a Dellorto fuel pump.

7) Purchase and install a ME-Shifter F1 – Paddle Shifting System.

There is just no time to conventionally bump shift this animal now !

8) Purchase and install a Model 315A V Force 8 pedal reed cage box

8) Purchase and install a high output programmable ignition system.

9) Have the cylinder ported and replated by a good engine builder like Eric Gorr of Eric Gorr Racing.

10) Use only Wiseco pistons.

11) Make or buy a seat side cranking system for easier engine starting.

I know all the above engine modifications are very,very pricey….but going fast and racing cost lots of money !

But we did all the above Mods. and it was worth every penny we feel.

Besides…What could be more fun on four wheels then zipping around with a 250 on a nice afternoon ?

These results were after hours & hours of fabrication and trackside testing….and they all work really well.

I know I’m very vague with the instructions ..so instead carefully study these inc. photos for more additional clarifications of the work to be done.